In recent years, data traffic has rapidly increased due to the increased use of smartphones. Smartphone users will continue to increase in number and application services using smartphones, such as SNS, game, etc., will also be more varied. Therefore, far more data traffic will also be required to cope with this increased demand. In particular, Machine-to-Machine communication using things, beyond communication between people, is expanding in mobile communication markets. Examples of the M2M communication are communication between human and things, communication between things, etc. When M2M communication is normally used, traffic transmitted to an eNB will increase to such an extraordinary extent so that it cannot by processed by the eNB.
In order to resolve these problems, a direct communication technology between devices, also called Device to Device (D2D) communication, has been created. The D2D communication technology is created by both a licensed band of mobile communication and an unlicensed band such as wireless LAN.
When D2D communication is combined with mobile communication, this is advantageous in that evolved Node B (eNB) increases traffic accommodation capability and reduces overload. In D2D communication, User Equipment (UE) devices, within the same cell or cells adjacent to each other, establish D2D links with each other and exchange data with each other, via the D2D links, without eNBs. Therefore, D2D communication can reduce the number of links from two to one.
Studies on D2D communication in an unlicensed band are to: detect requests created in Human-to-human communication, human-to-machine communication, and machine-to-machine communication; prevent wastage of unnecessary wireless resources; properly determine locally created traffic; and provide corresponding services. That is, D2D communication in an unlicensed band has been studied, focusing on a method for a number of devices to broadcast services and information regarding content and efficiently receive corresponding services and information.
Unlike existing ad-hoc/sensor networks, D2D communication devices first synchronize with each other and then perform discovery, pairing, scheduling, etc. Device-to-Device Synchronization allows devices performing D2D communication to: perform efficient transmission/reception of data or control signals; and configure an efficient protocol for scheduling. Existing communication techniques have difficulty in creating efficient distribution protocol to control networks without a master node; however, D2D communication allows devices to synchronize with each other, so that distributed devices can efficiently exchange control signals with each other, which can be considered distribution protocol.
Distribution scheduling refers to a method which is performed in such a way that allocation of resources is determined, depending on part of information in individual local areas, but not in such a way that one master obtains information regarding the entire network channel, etc. and allocates resources. Therefore, it is not easy to achieve the maximum scheduling capacity. Accordingly, a method is required to achieve the maximum efficiency and to minimize overhead by control signals when performing distribution scheduling.
Proximate discovery related to D2D communication has been implemented with various methods, e.g., a method using short-range communication or Over-The-Top (OTT). OTT refers to a method performed, based on existing Internet, in such a way that: an Application Service Provider (ASP) collects information related to services and locations via application programs of UE, performs a matching procedure to determine whether individual UE devices are located within a service receivable range, and informs the UE devices of information according to the matching result, thereby allowing the UE devices to perform discovery. A method using short-range communication, such as Wi-Fi, Bluetooth, or the like, is performed in such a way that: UE embedded with service discovery protocol performs proximate discovery, but networks do not control proximate discovery. The method using short-range communication has generally been used to establish a short-range connection.
The conventional OTT is a method suitable for discovering information regarding devices which are fixed in location. In OTT, mobile devices need to periodically transmit information regarding their locations and recognition information regarding users/services to a server, in order to identify locations of each other and search for related services, and to receive a related matching result from the server. Therefore, OTT requires a relatively large amount of network resources. In addition, since a mobile device needs to measure its location and transmit the measured location value to the server, it consumes electric power in the process of detecting its location using a second device, such as GPS, Wi-Fi, etc.
In order to resolve the problems described above, a system has recently been developed that has a fixed discovery repetition, considering a synchronous network based on GPS or cellular signals. Referring to FIG. 1, the system is implemented in such a way that: each of the discovery repetitions 101 and 103 is allocated a certain size of discovery intervals 105 and 113 and the resource within each of the discovery intervals 105 and 113 is configured with a number of discovery slots 107, 109, and 111 which have the same size. In order to select a discovery slot to be used for proximate discovery, UE observes a state of energy of transmission/reception signals of discovery slots 107, 109, and 111 during a discovery interval 105. After that, UE selects a discovery slot with a relatively low level of energy and transmits a discovery signal via the next discovery repetition 103.
The conventional system is operated in such a way that a new mobile communication system is allowed only after all mobile devices transmit discovery signals during the discovery interval and then the new mobile device detects states of energy of discovery slots and selects a discovery slot with low interference. However, in a real situation, discovery repetition varies according to types of services and all mobile devices may not transmit discovery signals during the discovery interval. Therefore, the system has a risk that may cause loss due to interference prediction error.